TAMPA — Freshmen and their parents came to campus for orientation. Students sat in biology class. An estimated 20,000 people were at the University of South Florida on Friday afternoon when the quiet calm of summer was shattered by a siren.

At 12:15 p.m., a male caller had contacted an operator at the Crisis Center of Tampa Bay.

He said he had a gun and wasn't afraid to use it.

Before he hung up, he said he was in a parking lot outside USF's Bio-Science Building.

In the two hours that followed, police established perimeters and combed the campus, guns drawn. Students, warned by a text message blast to stay locked inside, heard a police helicopter overhead and wondered whether this time the threat was real.

Investigators identified the caller as a black man, 5 feet 7, weighing 200 pounds, said Hillsborough County Sheriff's Office spokesman J.D. Callaway. They knew who he was Friday afternoon, but not how to get in touch with him.

Despite the inconveniences and the academic disruptions of what turned out to be a drill, such precautions are necessary these days, said USF police Maj. J.D. Withrow.

"We can't let our guard down," Withrow said.

• • •

Just last month, police received reports of a man with a pistol in the campus' Greek Village area. A search found no one.

And in June 2008, the campus went on alert until authorities learned that a reported gunman was an ROTC student carrying a fake rifle.

But there are those cases that turn out to be the real thing:

The man arrested for pointing a gun at a crowd of students at a talent show last March.

After the 1999 massacre at Columbine High School in Colorado, his department began active shooter training. The department had a drill just two weeks ago.

And after the 2007 rampage at Virginia Tech, the number of students and employees signed up for MoBull, a USF emergency text-messaging system, swelled. Once just a couple of thousand, that number now stands at 55,800.

In January 2008, the school hired a vice president of public safety. And this summer, officials installed speakers with strobe lights across campus to broadcast emergency messages.

The system was first used Friday.

• • •

Before the alarm sounded, student Nathan Terryman, 21, left biology class to use his cell phone.

He tried to get back into the Bio-Science Building, but the door was locked. A police officer told him to leave.

He was worried, he said as he walked away. His girlfriend was still inside.

In a nearby building's chemistry class, Sai Vankayala, 29, was too close to Bio-Science to leave. He stayed inside for two hours, hungry and scared.

Another young man approached a group of officers gathered outside the library. After a brief exchange, he ran across the parking lot, ducking the whole way.

And on the Internet, the alarm rang just as loud. Twitter.com was abuzz with updates.

TeamAmeriFace: needs prayers, wife stuck on a campus that has a gunman loose on it …

anupampradhan:… We are presently locked inside the building … watching 5 hovering choppers.

jdubindustries: I wish the gunman on USF campus would go away. These sirens are irritating.

The social networking site provided a forum to pass along news, but false updates created confusion. Some users prematurely gave the all clear, linking to articles from an incident in June. Others continued with warnings, long after the threat had subsided.

At 2:12 p.m., a second text blast went out on MoBull: Police have investigated area. Okay to resume normal activities.

Hillsborough County sheriff's deputies worked into the night trying to contact the man who had made the call, but they had not filed criminal charges.

Kim Wilmath, Michael Van Sickler and Steven Overly contributed to this report. Alexandra Zayas can be reached at azayas@sptimes.com or (813) 226-3354.

SKIP O’ROURKE | Times

A Florida Highway Patrol trooper gets ready to put his weapon away at USF’s Tampa campus Friday. After about two hours, the search for a gunman ended and authorities sounded an all clear.﻿